CSE 232 Spring 2003

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CSE 232 Spring 2003

• Notes on the color table and indexed color concept

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General idea

• Images can consume a large amount of storage
  space in main or disk memory
• Gray scale or labeled image needs only small
  integer pixels (0, 255 range is good)
• RGB color image can be 3X as large
• Can use a color table to convert an integer label
  L to color code RL, GL, BL
• Just need to make a table so that color codes
  can be obtained using index L

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Color table cheaper graphics

• image has 8-bit pixels
• each 8-bit number is an index into a color look
  up table
• can change colors without changing image

512 x 512 RGB image with 3x8-bit color values
requires 750K Bytes. 512 x 512 8-bit codes 256K
Bytes 256 x 3x8-bit color table 257K Bytes
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Color Table concept
int pixel image (P2)
Color table
RGB pixel image (P3)
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GIF image format imagetable (plus other stuff)

• header information
• color table
• image pixels (up to 8 bits each) LZW compressed
• RGB data is stored in the color table and NOT
  in the image pixels themselves
• get good compression and ability to quickly
  change the colors
• there are only 256 different colors for any
  single image, but table RGB triples can be
  changed
• gif files are very good for line drawings

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ColorTable ADT

• model is an ADT array of up to 256 colors
• array index is an int in 0, 255, which well
  call a pseudo color or color index
• operations are create, destroy, add new entry,
  (see P2 specs for others)

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Types of compression

• Lossless no information is lost every bit of
  the original image can be recovered.
• if an image has no more than 256 different color
  triples in it, then color table can exactly
  recreate it.
• Lossy information is approximated the original
  image cannot be recovered exactly.
• Suppose there are 1000 color triples in the
  original image but we replace each by the closest
  one of a set of 256 triples in color table
• e.g. (218, 58, 150) ? (220, 60, 150)